Ceramic matrix composites generally include a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications, such as in gas turbines, are silicon-based composites, which include silicon carbide (SiC) as the matrix and/or reinforcement material.
Different processing methods have been employed in forming CMCs. For example, one approach includes melt infiltration (MI), which employs a molten silicon to infiltrate into a fiber-containing perform. CMCs formed by MI are generally fully dense, e.g., having generally zero residual porosity. Another approach for forming CMCs is chemical vapor infiltration (CVI). CVI is a process whereby a matrix material is infiltrated into a fibrous preform by the use of reactive gases at elevated temperature to form the fiber-reinforced composite. Generally, limitations introduced by having reactants diffuse into the preform and by-product gases diffusing out of the perform result in relatively high residual porosity of between about 10 percent and about 15 percent in the composite. In particular, typically in forming CMCs using CVI, the outer portion of the composite formed by CVI typically has a lower porosity than the porosity of the inner portion of the composite. Another approach for forming CMCs includes initially employing a partial CVI process followed by a MI process. This approach usually yields lower residual porosity of between about 5 percent and about 7 percent.
There is a need for further ceramic matrix composites (CMC), and more particularly, to articles and methods for forming ceramic matrix composite articles having.